One of the most common mistakes I see on jobsites is operators treating all concrete the same. The way you operate a vibratory power screed must change entirely based on the "slump" of your mix. Slump is the measure of the concrete's workability and fluidity. A low slump (e.g., 75 mm [approx. 3 inches]) is stiff and dry, often used for sloped driveways or heavy structural slabs. A high slump (e.g., 150 mm [approx. 6 inches]) is wet and flows easily.
When I’m faced with a stiff, low-slump mix, the power screed is working at its absolute maximum capacity. I have to rev the engine high to generate maximum vibration amplitude just to get the aggregate to consolidate. The physical pull on the operator is immense because the machine wants to climb up and over the stiff mud rather than push it. You have to keep the blade angled sharply and make multiple passes to pull the moisture to the top.
Conversely, if the ready-mix truck delivers a high-slump, watery mix, you have to back way off the throttle. If you vibrate a high-slump mix too aggressively, you will cause "segregation." The heavy gravel will sink like stones in a pond, leaving nothing but weak, watery cement paste on the top 50 mm [approx. 2 inches] of the slab. When that slab cures, the surface will be brittle and will likely dust or flake off under traffic. Operating this machine is a constant balancing act between the machine's vibration frequency and the concrete's internal chemistry.
